A diaphragm-type carburetor comprises a main body portion defining a carburetor mixing passage having an air intake side and an engine outlet side, fuel pump means, a throttle shutter mounted within the carburetor mixing passage between the air intake side and the engine outlet side, a throttle shaft for controlling the throttle shutter, and a metering chamber for supplying fuel from the fuel pump means into the carburetor mixing passage via a high speed adjusting screw and a low speed/idle adjusting screw.
In such a carburetor the volume of fuel delivered to the engine is adjustable, for low speed operation via the low speed/idle adjusting screw and for high speed operation via the high speed adjusting screw. Adjustment is factory set by the engine manufacturer to give the desired engine performance/air fuel ratios.
With such a system, adjustment can be made within a broad band from no fuel flow, when the adjustment needle is screwed fully in (i.e. the needle tip closes the orifice) to fully open, when the needle tip is fully out of the orifice. In this case the orifice diameter controls the maximum volume of fuel flow. This system allows the engine to be set to run on a very lean or very rich fuel mixture. More often the correct factory setting is re-adjusted by the end user. Such lean and rich conditions result in undesirable exhaust emissions.
With the advent of emission regulations applicable to IC engines and in particular to two-stroke engines which are regulated by the type of carburetor mentioned above, it is necessary to have a system which limits the amount of adjustment on the low speed and high speed screw adjustment screws. It should also be possible for such a system to be adjusted and set after the carburetor has been assembled to a complete engine or a suitable end product such as typically, a chainsaw.
In particular with IC engine powered garden equipment, chainsaws etc., it is necessary for the manufacturer of the end product to final adjust set and certify air/fuel ratios to comply with emission regulations on each fully assembled unit prior to packing/shipping. This requires single point adjustment of the low and high speed screws, i.e. tuning each carburetor to each individual engine requirements prior to fixing the limits. It is not possible to achieve this with the above mentioned system.
It is an object of the present invention to overcome these problems by providing a device which will allow for optimum single point setting of both high and low adjust screws on a finished product such as a chainsaw or disc cutter prior to fixing the limited adjustment cap on the high speed and low speed screws.